Facilitation.

From Professor Starling’s Principles of Human Physiology I may again quote part of his account of Facilita­tion or “Bahnung.” “When an impulse has passed through a certain set of neurones to the exclusion of others it will tend, other things being equal, to take the same course on a future occasion, and each time it traverses this path the resistance in the path will be smaller. Education is the laying down of nerve-channels in the central nervous system, while still plastic, by the process of ‘Bahnung’ along fit paths combined with inhibi­tion (by pain) in the other unfit paths. Memory itself has the process of facilita­tion for its neural basis,” again, “stimula­tion of one anterior root produces no definite movement of a group of muscles, but partial contrac­tion of a number of muscles which do not normally contract simultaneously. Thus, stimula­tion of a sensory nerve may provoke either flexion or extension of a limb, not both simultaneously. Stimula­tion of the motor roots will cause simultaneous contrac­tion of both flexor and extensor muscles. It is this subordina­tion of morphological to physiological arrangements in the limbs which has necessitated the founda­tion of limb-plexuses.” (Italics not in the original). Professor Graham Kerr in his work on Embryology before mentioned says: “In early stages of Evolution, whether phylogenetic or ontogenetic, we may take it that vital impulses flitted hither and thither in an indefinite manner within the living substance and that one of the features of progressive evolution has been the gradual more and more precise defini­tion of the pathways of particular types of impulse, as well as the transmitting and receiving centres between which they pass. We may then regard the appearance of neuro-fibrils within the protoplasmic rudiment of the nerve-trunk as the coming into view of tracks, along which, owing to their high conductivity, nerve-impulses are repeatedly passing. It may be that as each successive passer-by causes a jungle-pathway to become more clearly defined so each passing impulse makes the way easier for its successors and makes it less likely for them to stray into the surrounding substance” (p. 112).

Professor Macdonald, in the Portsmouth address referred to, speaking of the states of the cells under excita­tion, rest, and inhibi­tion, says “excita­tion is associated with an increase in pressure of certain particles within the cells; in rest these particles are in their normal quantity and have their normal number. During inhibi­tion they are decreased in number or have a retarded motion. Thus it happens that the excited cell tends to grow in size, on the other hand the inhibited cell tends to diminish, and the resting cell to remain unaltered in the nervous system. Structure is everywhere the outcome of function.” Speaking of the relationship of parts within the nervous system, “In so far as it is fixed, it is a sign of the orderly action of circumstance upon the structures of the body, and the result rather than the cause of the monotony of existence. I hold it as probable that all the individual structures of the nervous system, and so in the brain, have just so much difference from one another in size and shape and in function as is the outcome of that measure of physical experience to which each one of them has been subjected; and that the physiological function of each one of them is of the simplest kind. The magnificent utility of the whole system, where the individual units have such simplicity, is due to the physically developed peculiarities of their arrangement in relation to one another, and to the receptive surfaces and motor-organs of the body.” As to the lens-system of the eyeball he remarks, “Surely there is no escape from the statement that either external agency cognisant of light, or light itself has formed and developed to such a state of perfec­tion this purely optical mechanism, and that natural selection can have done no more than assist in this process.” He applies the same conclusion to the formation of the sound-conducting and resonant portion of the ear as well as the semi-circular canals and to the cerebellum. These statements are not strictly associated with this chapter but bear by analogy very strongly on the matter at issue. Indeed the whole of this address might be utilised by a junior counsel for Lamarck if he rested alone on the authority of a leading physiologist. The same may be said of the anatomist whose Arboreal Man has attracted so much attention. Speaking of the arboreal habit in the phylogenetic history of mammals he asks the question, “How did this factor enable that particular stock to acquire supremacy?” and says that it will be answered as far as it is possible, by the study of the influence of the arboreal habit upon the animal body; which may be put in another way as the produc­tion of reflex-arcs suited thereto (p. 3.) Of the muscle groups of fore and hind limbs he says, “With a simple arrangement of anatomical parts a slight shifting of muscular origins has turned a perfectly mobile second segment into a supporting segment constructed upon very simple lines: that these changes are those produced by the demands of support from the hind-limbs in tree-climbing seems obvious” (p. 6); of the position of uprightness upon a flexed thigh of an arboreal man, “It is tree-climbing which makes this posture a possibility” (p. 63). “But it is not to be doubted that the underlying principle is clear enough, that the arboreal habit develops the specialised and opposable thumb and big toe” (p. 71). “Even before the power of grasp is developed, we may imagine the dawn stages of educational advances initiated by hand-touch” (p. 159). “Tactile impressions gained through the hand are therefore perpetually streaming into the brain of an arboreal animal and new avenues of learning about its surroundings are being opened up as additions to the olfactory and snout-tactile routes” (p. 160). He asks also the pertinent question, and says at least a partial answer to it can be given, “Did the cerebral advance create the physical adaptations, or did the physical adaptations make possible a cerebral advance?” (p. 196). Two more statements from this chapter show what the answer to this question from the anatomist would be—“and again in the evolutionary story we are forced back to consider a combina­tion of seemingly trivial, and apparently chance associations: in this case the dawning possibilities of neo-pallial developments combined with the physical adaptations due directly to environmental influences” (p. 198). I have ventured to underline this passage.

I regret the necessary length of these quotations but, on account of them, can the better be suffered to finish this study, when I briefly consider certain well-known nervous reactions in the cat and dog as to their probable origin. It would be a highly interesting thing to hear an exposi­tion by an expert of all the reflexes and reflex-arcs of such a system as those which in a cat, dog, ape, or man are concerned with the passage of a morsel of food from the mouth through all its chequered and varied career till it undergoes metabolism and excretion, but I could not do it if I would, and would not here if I could, because of their fundamental fixed and innate character, and I think it simpler and safer to refer to such minor reflex-arcs as those which govern the scratch-reflex in a dog, the pinna reflexes in a cat, and a few smaller ones, on the principle of ex uno disce omnes. Such minor nerve-mechanisms as these in a pair of well-known domesticated animals will suffice for evidence on behalf of initiative in evolution.